Seed phrase
In short A seed phrase is a 12- to 24-word backup that lets a compatible wallet rebuild the private keys and accounts derived from it.
A seed phrase, also called a recovery phrase or mnemonic, is a list of words your wallet generates when you create it. Most wallets use 12 or 24 words, though the BIP-39 standard also allows 15, 18, and 21. From those words, a wallet can rebuild every private key derived from them. Whoever holds the words controls the funds, and nobody else, including the wallet's developers, can restore them for you.
The words encode a large random number, with each word picked from a fixed list of 2,048. The wallet turns that number into a root seed and derives a tree of private keys from it along set paths. Because this is deterministic, entering the same words into another compatible wallet produces the same addresses. One phrase can cover Ethereum, BNB Chain, Base, and Solana accounts, although wallets sometimes use different derivation paths and show different addresses for the same words.
Your tokens never sit inside the app. The app is an interface to keys, and the phrase is the standard way back in if your phone breaks, though another device where the wallet is still unlocked, or a separate private-key backup, can also work. Accounts you imported by private key are not derived from the phrase, so they need their own backup. If someone else sees your phrase, they can import it on their own device and sweep everything, and blockchain transfers cannot be reversed. Connecting to a site or approving an ordinary transaction does not reveal the phrase, but a malicious approval can still let a contract drain your tokens, which is a different risk.
Write the phrase on paper or stamp it in metal, and keep it offline, ideally in two separate secure places. Never photograph it or store it in notes, cloud drives, email, or chat. BIP-39 also allows an optional passphrase, sometimes called a 25th word, which produces a completely different wallet. If you use one, recovery needs the words and the passphrase, so back up both.
A wallet asks for your phrase only when you deliberately choose to restore or import, inside the official app or on a hardware device you got from the official source. An unsolicited request is a scam, whether it comes from a website, a support agent, an airdrop claim, or a pop-up. Also be wary of a wallet that arrives with a phrase already written on a card, since the seller may have kept a copy.